An IC-compatible oscillator is provided with an operational amplifier having an AFC function (Automatic Frequency Control function). Namely, a resistance value of the operational amplifier is controlled so as to arbitrarily change a gain and a reference value of an external voltage. Thus variation of electrical properties among respective ICs has been adjusted so as to obtain an AFC property corresponding to a plurality of specifications demanded by customers with one type of IC. FIG. 11 is a diagram showing a circuit including an operational amplifier having related art AFC function. In this AFC circuit, a control voltage from a VCONT terminal is inputted to an operational amplifier 30 constituting an AFC gain adjustment circuit 34. A gain adjustment of the operational amplifier 30 is conducted by adjusting a resistance R10 in accordance with memory data stored in a memory. A reference adjustment of the AFC circuit is conducted by an operational amplifier 31 constituting a reference adjustment circuit 35. A reference adjustment of the operational amplifier 31 is conducted by adjusting a resistance R1 in accordance with memory data stored in the memory. These voltages are amplified by an operational amplifier 32 to be VAFC, being applied to a varactor 33 of an oscillation circuit 36.
[Patent Document 1] Japanese Unexamined Patent Publication Application No. 2005-303388
[Patent Document 2] Japanese Unexamined Patent Publication Application No. 2006-033092
[Patent Document 3] Japanese Unexamined Patent Publication Application No. 2006-086679